In a communication network of distributed type, provision is made for sharing digital data, such as audio files, video sequences, images, animations, etc. between several communication apparatuses connected to the network.
For example, in so-called peer-to-peer communication systems, it is allowed to have large spaces for storage of data and to exchange multimedia data of large size.
It should be noted that, in a distributed network of peer-to-peer type, such as the Internet network, the communication apparatuses connected, which are for example computers, may be considered either as clients, in which case they are adapted to receive data, or as servers, in which case they adapted to provide data to other apparatuses of the network.
However, in such a network, the apparatuses are not connected permanently to the network, which means that all the data are not accessible at every instant.
Furthermore, at each new connection, the address on the network of the apparatuses may change. Thus the problem of finding the apparatuses arises.
One solution to this problem consists of having recourse to a server, termed central server, of which the role is to ensure a minimum quality of service. To do this, the server may store in memory, in particular in a temporary memory of cache memory type, a portion of the data exchanged between the different apparatuses of the communication network so as to enable access to those data if no communication apparatus suitable for delivering those data is connected to the network.
Such a network is then termed a hybrid peer to peer system.
The transmission of data, in particular multimedia data such as video, involves the transmission of large quantities of data. For this, the use of the bandwidth and of the storage resources must be optimized.
One approach consists of exploiting the different coding formats available so as to code the multimedia data while providing the best possible quality and a minimum quantity of data.
Among the different video communication formats is the MPEG1 standard used in particular for coding and storing videos on a carrier of CD type, the MPEG2 used in particular to code and store videos on a carrier of DVD type or for coding digital television broadcast by satellite. Finally, the MPEG4 standard makes it possible to code video in formats that are very widespread on the Internet such as DivX and Xvid.
Other standards are also in course of standardization, such as the H264 and SVC standards making it possible to further improve the rates of compression obtained by the MPEG4 standard by a factor of 1.5 to 2, while at the same time providing a better display quality.
However, when an item of data is not present in a format adapted to be transmitted, the change in format needing to be carried out necessitates a long coding time.
Conventionally, coding of multimedia data is considered as fast, but on a video of one hour this represents an hour of conversion which is too long to be carried out during the communication steps.
Thus, such coding making possible a change in format is incompatible with a need to rapidly provide a representation of the original content of the multimedia data.
In conventional data sharing systems, all the data is loaded onto the communication apparatus which requires the data, that is to say the recipient communication apparatus. Once the loading of the data has been carried out, these latter become available both for being made accessible to the other terminals of the network in the peer-to-peer network, and for being viewed by the user.
In the case of loading video data, the size of those data is such that the downloading time is incompatible with an interactive use of those data.
Thus, the user is obliged to launch the downloading of the data and then wait for several hours for all the data to be downloaded and stored in order to view the video sequence at last.
Another approach consists of viewing a streamed video sequence in real time. With this mechanism, the user loads and views the video data without however storing those data. This solution however has the advantage of minimizing the waiting time of the user.
According to this approach, the client only receives a portion of the data in order to display them, but not the entirety of the information. This is because certain portions may be lost during the transmission to the user without them however being resent. To adapt the transmission of the video to the available bandwidth, the latter is modified by reducing the quality, or the resolution or by deleting certain images. On account of this, it is not possible to store those data and to make them available to the other users.
Such a system is adapted to interactive applications but it is not adapted to the downloading and storage of the data in a peer-to-peer type network.
These approaches also have another drawback. This is because they do not give the user the possibility of rapidly identifying portions of the content of the video data which are of interest to that user.
Thus, the user is obliged to view the whole of the video sequence to identify the portions which he wishes especially to access.
Thus, the main problem lies in the absence of a system making it possible to rapidly transmit and exchange, between terminals of a peer-to-peer type network, multimedia content or portions of multimedia content of video sequence type, while ensuring the best possible video quality, that multimedia content being constituted by large quantities of data.
Moreover, to that problem there is added that of avoiding the duplication of multimedia content at the time of the transmissions between the communication apparatuses of the peer-to-peer network in order to save bandwidth.
Numerous studies have been conducted over several years with the object of minimizing the impact of the transmission and of the loading of video sequences and thus on the access by the users to those video sequences.
There is known, in particular, from U.S. Pat. No. 6,166,735 of International Business Machines Corporation (IBM), a system in which multimedia content, and more particularly videos, may be downloaded selectively onto a client communication apparatus, from a remote communication apparatus, the server. This is a client-server system.
This system enables the user to download, then view representation points of the original stream identifying sections of the video sequence. Each representation point is shown by a thumbnail image illustrating the section of the video sequence. On selecting one or more representation points, the user may thus obtain and rapidly access the section of the original video sequence identified by selected representation points.
However, the user only has slight information for cognition of the content of the video section corresponding to the thumbnail image, and this system only partially solves the problem of transmission and of reduction of the bandwidth.
Thus, these methods have the drawback of both not transmitting multimedia data rapidly and not optimizing the use of the bandwidth on the network.
It would consequently be desirable to be able to reduce the quantity of information transmitted, in particular by making an appropriate selection of the information to transmit.